1. Field of the Invention
The present invention provides an improved circuit for use in use respirator type apparatus wherein the operating point of the apparatus is stabilized over large temperature ranges.
2. Description of the Prior Art
Apparatus for supplying oxygen to a patient in response to the patient""s breathing cycles have been available in the prior art. In such devices, the breathing cycles are sensed by a capacitive pressure sensor, or transducer, which provides an electrical output signal corresponding to the breathing cycle. An example of such an apparatus is shown in U.S. Pat. No. 4,612,928 to Tiep et al and U.S. Pat. No. 5,134,886 to the inventor of the present invention. Capacitive transducers however are difficult to manufacture and tend to be hand fabricated, increasing the costs associated therewith. In addition, capacitive transducers tend to drift and fail over time.
In order to overcome the disadvantages of capacitive transducers, efforts have been made to use piezoresistive sensors as a replacement since they are readily available, inexpensive and reliable. In this case, a conventional differential amplifier provides a constant DC voltage output in response to a constant pressure input from the transducer. The amplifier typically has a voltage gain of 7000 or greater in order to detect a momentary nasal inspiration pressure of approximately 0.002 psi. However, it is difficult to maintain the stability of the pressure sensor at this high voltage gain over a large temperature range. Although the sensors can be individually temperature compensated through testing, it is not practical for low cost production requirements.
What is desired is to provide an improved circuit for use in respirator type apparatus wherein a piezoresistive transducer is utilized to sense patient breaths and wherein temperature effects on circuit stabilization are minimized.
The present invention provides a high sensitivity circuit for use in respiration type apparatus using a piezoresistive transducer and wherein the temperature effects on system stabilization are minimized. In addition, means are provided to reduce system initialization times. The apparatus can be designed to regulate the patient intake of oxygen or medicines, such as insulin and respiratory medicines.
The circuit comprises a differential sensor amplifier which responds to the output of the piezoresistive transducer, a dc negative feedback circuit connected to the output of the differential amplifier, means for reducing the initialization time of the circuit, circuit means for clamping the voltage output and a voltage comparator for tracking the circuit operating point and switching its output state when a sudden voltage change caused by a patient breath is detected.
The present invention thus provides a simple, reliable and inexpensive circuit for stabilizing the temperature effects in a respiration type apparatus using a piezoresistive transducer.